One of the major problems with cancer treatments is the fact that they do not target the cancer cells specifically without harming the rest of the body. Therefore, the development of inhibitors against proteins that are overexpressed in breast cancer (e.g. Grb7) will be important in developing improved therapeutics with increased specificity and less toxicity. Grb7 is a promising target for therapeutic intervention as it is likely to play a direct role in expression of malignant phenotypes such as cell migration, survival, and invasion, especially in breast and esophageal cancers. Grb7 maps closely to the ErbB2 gene and is frequently co-amplified and overexpressed with ErbB2, an important breast cancer marker associated with poor prognosis. Another promising feature of Grb7 as a tumor target is its limited tissue distribution. Grb7 has been found overexpressed in esophageal cancer tissue, while adjacent normal tissue does not express detectable amounts of Grb7. Using phage display, we have identified cyclic peptides that bind to Grb7. These peptides bind specifically to the Src homology-2 (SH2) domain of Grb7 with no detectable binding to Grb2, even though Grb2 also selects for Asn at pY+2, or to Grb14, a closely related family member of Grb7. To increase the stability of the peptide in preparation for cell studies, a thioether version of one of the non-phosphorylated peptides (G718NATE) was synthesized. This analog was shown to effectively inhibit the association of Grb7 with the ErbB family of receptor tyrosine kinases in breast cancer cells that overexpress Grb7. The goal of this proposal is to develop G718NATE and newly identified peptides into a cell permeable molecule to inhibit Grb7 function and ultimately inhibit the progression of cancer. The specific aims of this renewal application are (1) Develop a Grb7 binding analog that translocates across the cell membrane. (2) Determine the effects of the cell permeable Grb7-binding peptides on an array of important functional parameters of cells overexpressing Grb7. (3) Use the structure-based, homologous analog and biased library approach to make educated designs to increase the affinity of the Grb7 ligands